The Advantages of Polypropylene Piping for HVAC Applications

Polypropylene heat fusion connections join the pipe and fitting together at a molecular level, so there is no physical difference between the actual connection and the pipe itself.

At this home, polypropylene was used to connect storage tanks to a lake plate heat exchanger, a heat pump, a radiant floor system, and other elements. The pipe was selected on this job because the owners liked its environmental friendliness and the fact that it is chemically inert, in addition to other reasons.

The heat fusion welding process when properly executed is nearly leak-proof because the pipe and fitting become one. This cutaway of a main pipe, a fusion outlet, and a secondary pipe shows that there are no leak paths within the pipe assembly.

“We do it that way because we’ve always done it that way and it works.” You might hear this refrain frequently, and you might use it yourself, and there’s certainly nothing wrong with that.

In fact when a system is proven and you’re comfortable with it, it can make sense to stick with it. However, every now and then it’s wise to investigate and embrace new technologies.

In the HVAC contracting world, polypropylene pipe is just such an exception. While it’s often viewed as “new” and lumped in with other plastics, it’s in fact been used around the world for nearly 40 years and is highly engineered — offering HVAC application benefits that many other plastics cannot. It also offers several advantages over metal HVAC pipe systems.

Polypropylene pipes have been used for years on HVAC and related systems of all types:

• Chiller installations;

• Radiant heating and cooling systems;

• Data center cooling;

• Geothermal heat pump earth loops;

• Snowmelting;

• And several others, some beyond the scope of this article.

Stable Pricing and No Hacksaw-Wielding Thieves

How many times have you bid a job and by the time the bid’s accepted and materials are to be ordered, the price of metal pipe and fittings has jumped up significantly? How many times has 24/7 jobsite security had to be factored into the bid? The price of polypropylene pipe remains quite stable, and it’s rarely, if ever stolen from a jobsite.

In addition to these benefits, the polypropylene heat fusion welding process combined with a much lighter weight than metal can significantly reduce labor costs. Heat fusion welding is a safe and flameless technology that has proven successful worldwide for nearly four decades.

Heat Fusion is the Key

Heat fusion is much stronger and more reliable than other forms of chemical bonding (gluing, chemical, welding, etc.) and can be used almost immediately, rather than having to wait overnight for the connection to set. It is generally either much quicker or much more cost effective than most mechanical forms of bonding, and tends to be more reliable because heat fusion does not introduce any foreign materials into the system. The obvious advantage over a soldered system is that there are no flames or fumes with heat fusion, and it also tends to take only a fraction of the time to complete a connection.

The electric welding iron plugs into a standard 110V outlet and heats up to 450-550°F. Special welding heads, designed to match various sizes of pipes and fittings, are attached to the welding iron. These heads increase the surface contact area, making the process much faster than traditional butt fusion (a process commonly used with polyethylene and large-size polypropylene).

The installer gently pushes the pipe and fitting onto the welding heads and allows them to heat up for a factory-specified amount of time (typically only seconds). He then removes the pipe and fitting from the welding heads and slides the pipe into the fitting. The pipe and fitting are allowed to cool together under slight pressure. As they cool, the polypropylene from the two different pieces mixes together, forming a bond that is stronger than either the pipe or the fitting alone.

These connections, when formed properly, will never leak or break because they have the exact same composition as the pipe and are much thicker. The electric iron does not release any fumes, toxins, VOCs, or smoke while operating — allowing the installer to make all his connections without jeopardizing his health, safety, or the site’s IAQ.

Polypropylene also lends itself to the installation of fusion outlets, which can be huge labor savers since they allow installers to simply drill into the supply pipe, then heat fuse the fusion outlet fitting into place wherever needed. A single fusion outlet can be used instead of a reducing tee, replacing two long connections with a single, 30-second fusion.

The fittings are also very cheap on average (transition fittings cost more, but using fusion outlets instead of reducing tees costs much less), making the overall cost per connection extremely competitive, and often much lower than other piping systems. Fusion outlets are popular with maintenance personnel since they allow for easy expansion and adaptation of HVAC piping systems.

While there is a learning curve to working with polypropylene, pipefitters and those already familiar with heat fusion are usually able to run with it very quickly. A half-day of training typically gets even the most inexperienced of crews ready for action and labor savings of 25-30 percent are possible.

‘How Does it Hang?’

Polypropylene-random (PP-R) offers thermal expansion benefits other plastic piping systems used for HVAC applications cannot. With an optional fiberglass extruded layer, it is resistant to thermal expansion by up to 75 percent compared to other plastic piping, which ensures it hangs rigidly in operating temperatures up to 200°F.

As pipes change temperatures, they change in length. This simple law of physics has frustrated engineers for centuries. The change in length can result in tremendous forces being generated within the system and significant movement of the piping system. The combined effect of forces and movement often has resulted in catastrophic failures. To avoid this, engineers have typically resorted to either expansion loops or expansion joints. These methods of compensating for thermal expansion add cost and complexity to piping systems.

Historically pipes have been securely anchored with expansion compensation in place before the hot water is allowed to flow through them. When the flow starts, the hot water enters the pipe, and it grows to its new length. A new approach is now possible. With polypropylene technology, it is possible to run the warm water through the pipe and then fix the pipe supports. During normal operation, this eliminates thermal expansion concerns. During times when the heating water is off, the pipe is allowed to cool back down, and will try to contract and be under tension.

Polypropylene with a fiberglass extruded layer reduces this contraction by 75 percent. The remaining contraction results in relatively low tension stress that is absorbed by the fixed pipe supports. While being under tension could result in failures or leaks with many pipe joining systems, it is not a concern with the fusion fittings.

No Rust or Corrosion and Natural Insulation

Since polypropylene is rust-proof and corrosion-, scale-, and chemical-resistant it can often negate the need for chemical treatments. More importantly for HVAC applications, it is resistant to additives such as polyolester (POE) oil. POE oil is an ester-based grease commonly used in mechanical system components as a lubricant. It can wash out of the components and coat the inside of pipes, causing a phenomenon known as environmental stress cracking.

On top of all these benefits, polypropylene also offers a natural R-value of 1 and an excellent flow rate. The inherent insulation value in some cases allows a system to be installed without insulation, and can at minimum reduce the amount of insulation needed. The flow rate allows for better system efficiency and longevity.

In Conclusion

Overall, heat fusion is one of the best choices an HVAC piping installer can make. It’s faster and safer than soldering or welding; cheaper and more reliable than mechanical fittings; and stronger and more consistent than chemical connections with plastics. It’s also one of the most environmentally friendly piping options. So rather than sticking with what’s always worked, maybe it’s time to investigate a new HVAC piping option that could set your business apart and boost your bottom line.

Publication date: 01/23/2012

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Steve Clark has worked as a professional engineer (P.E.) in the United States, Canada, and the United Kingdom. He has worked as a development and applications engineer for the Trane Company and as an HVAC and energy engineer for consulting engineering companies, including his own firms, with an emphasis on building energy efficiency. He currently serves as president for North America for the German-made polypropylene-piping manufacturer Aquatherm. He can be contacted at steve.clark@aquathermpipe.com. For more information about Aquatherm, visit www.aquathermpipe.com.